1. Field of the Invention
The present invention relates to a high-vacuum die-casting method, and more particularly, to a high-vacuum die-casting method capable of efficiently discharging a gas in a cavity while a molten metal is injected into the cavity in which a metal product is to be molded.
2. Description of the Related Art
A die-casting method used for mass producing metal products commonly includes a series of processes of keeping a movable mold in contact with a fixed mold so that a cavity in which a metal product is to be molded is formed, injecting a molten metal into the cavity so that the molten metal is filled in the cavity, and, when the molten metal filled in the cavity is hardened into a metal product, separating the movable mold from the fixed mold and having an eject plate provided on a rear surface (an opposite surface to a surface that faces the fixed mold) of the movable mold approach the movable mold so that the metal product is ejected from the movable mold by eject pins fixed to the eject plate. While the molten metal is injected into the cavity, when a gas (air or a gas obtained by pyrolyzing a release agent) that exists in the cavity or a gas injected into the cavity with the molten metal is not discharged to the outside but mixes with the molten metal, incomplete filling defects such as porosities and pin holes are generated in the metal product so that the strength of the metal product is deteriorated and, in a case where the metal product is welded to another product, welding quality is deteriorated.
Therefore, when a metal product in which an amount of the gas mixed with the molten metal in the cavity must be extremely small, for example, an aluminum product that must have high tensile strength, high yield strength, and high elongation and that must be welded to other parts such as an aluminum front pillar (referred to as an A pillar) and a shock-absorber case used for a vehicle is die-casted, a method of vacuum absorbing the gas that exists in the cavity and the gas that flowed into the cavity with the molten metal to discharge the vacuum-absorbed gases to the outside so that the molten metal is injected while maintaining the cavity to be vacuous is used. When only the gases (the gas that exists in the cavity and the gas that flowed into the cavity with the molten metal) in the cavity are vacuum absorbed to be discharged to the outside, it is difficult to prevent a gas on a side of the eject plate from flowing into the cavity through gaps between the eject pins and the movable mold by vacuum pressure formed in the cavity so that the above-described effect of discharging the gases in the cavity is deteriorated.
A known method to solve the above-described problems is a method of providing a cover on the rear surface of the movable mold so that a space in which the eject plate and one of the ends (the ends combined with the eject plate) of the eject pins are arranged is sealed up by the cover and, when the gases in the cavity are vacuum absorbed, simultaneously vacuum absorbing a gas in the space sealed up by the cover, and discharging the vacuum absorbed gas in the space sealed up by the cover to the outside through a different path from a discharge path of the gas in the cavity so that it is possible to prevent a gas outside the cavity from flowing into the cavity through the gaps between the eject pins and the movable mold.
In the method of simultaneously discharging the gases in the cavity and the gas in the sealed space, the gases in the cavity may be more smoothly discharged than in a method in which only the gases in the cavity are vacuum absorbed and the gas in the sealed space is not additionally vacuum absorbed. However, it is necessary to improve the efficiency of discharging the gases in the cavity.